(Reuters) - Japan has asked nuclear superpower Russia to send a special radiation treatment ship used to decommission nuclear submarines as it fights to contain the world's worst atomic crisis since Chernobyl, Japanese media said late on Monday.

Japanese engineers at the Fukushima Daiichi nuclear plant have been forced to release radioactive waste water into the sea. At the same time they are resorting to desperate measures to contain the damage, such as using bath salts to try to locate the source of leaks at the crippled complex 240 km (150 miles) north of Tokyo.

Three weeks after a 9.0 magnitude earthquake and massive tsunami hit northeast Japan, sending some of Daiichi's reactors into partial meltdown, engineers are no closer to regaining control of the power plant or stopping radioactive leaks.

The quake and tsunami left nearly 28,000 people dead or missing and Japan's northeast coast a wreck.

The world's costliest natural disaster has caused power blackouts and cuts to supply chains and business hours. It is threatening economic growth and the yen, while a recent opinion poll suggested voters want embattled Prime Minister Naoto Kan to form a coalition in order to steer Japan through its worst crisis since World War Two.

Plant operator Tokyo Electric Power Co (TEPCO) was forced on Monday to release low-level radioactive seawater that had been used to cool overheated fuel rods after it ran out of storage capacity for more highly contaminated water.

A TEPCO official was in tears as he told a news conference: "We are very sorry for this region and those involved."